The present invention relates to compositions and methods for treating subterranean formations. In particular, the present invention relates to encapsulated breakers and methods for breaking viscosified treatment fluids utilized in the treatment of subterranean formations.
Viscosified treatment fluids are used in subterranean formations for a variety of applications. These subterranean viscosified treatment fluids include fracturing fluids, drilling fluids, diverting fluids, and gravel packing fluids. Subterranean viscosified treatment fluids often utilize gelling agents that can increase their viscosity. Biopolymer and synthetic polymers are two of the most common types of gelling agents used in subterranean applications. More specific examples of gelling agents include, but are not limited to, galactomannan gums, such as guar and locust bean gum, cellulosic polymers, and other polysaccharides. These gelling agents can increase a fluid's viscosity which may be important for a number of reasons in subterranean applications. For example, an increase in viscosity is important for transferring hydraulic pressure to divert treatment fluids to another part of a formation or for preventing undesired leak-off of fluids into a formation from the buildup of filter cakes.
While it is important to monitor and increase the viscosity of a viscosified treatment fluid, it is often desirable to reduce the viscosity of a viscosified treatment fluid at a later time. For example, in the case of fracturing fluids, a sufficiently high viscosity is needed to retain the proppant particulates in suspension or at least to reduce the tendency of the proppant particulates to settle out of the fracturing fluid as the fracturing fluid flows along the created fracture. Once the fracturing fluid has performed its intended function, it is often desirable to lower the viscosity of the fracturing fluid to allow flow back of the spent fracturing fluid or following a treatment as part of the fluid-disposal process. The removal of the spent fracturing fluids from the subterranean formation is typically required to allow hydrocarbon production.
This reduction in viscosity of the fracturing fluid is often achieved by introducing a breaker to the fluid that breaks the cross-linking bonds of the polymer gels. A breaker can be incorporated into the fracturing fluid. However, this technique can be unreliable and sometimes results in incomplete breaking of the fluid and/or premature breaking of the fluid before the fracturing process is complete. It is generally desirable to break the fracturing fluids as soon as possible to start the recovery of hydrocarbons without prematurely breaking the fracturing fluid, which can decrease the number or length of fractures obtained and thus, the amount of hydrocarbon recovered.
Furthermore, commonly available breakers such as enzymes often degrade at moderate to relatively high temperatures. On the other hand, oxidizers used at relatively high temperatures often break gels at narrow temperature ranges which leads to rapid decrease in gel viscosity before completion of a subterranean operation. For example, sodium persulfate breaks gels at moderate temperatures (65° C. or 149° F.) and thus, its utility in controlled fracturing operation at higher temperature is limited. While encapsulation techniques have been used to form protective coatings around breakers, commonly available encapsulated breakers typically only work at low to moderate temperatures. For example, encapsulated sodium persulfate (commercially available as OPTIFLO II™ and OPTIFLO III™ from Halliburton Energy Services, Inc.) works only up to about 52° C. (125° F.) and 93° C. (200° F.) respectively.